The invention pertains to image acquisition and, particularly, to the control and readout of images using charge coupled devices. The invention has application in video cameras and other image acquisition devices used in machine vision and in other industrial, research and recreational environments.
Manufacturers of video cameras and other digital image acquisition devices are increasingly reliant on charge coupled devices (CCD""s) to capture images for real-time processing. These devices are made up of hundreds or thousands of microscopic semiconductor elements arranged in closely spaced arrays. When the array is exposed to light (or other radiation), each of its constituent elements accumulate an electronic charge proportional to the number of photons that strike it. By focusing a scene on such an array, an electronic image is formed with brighter regions of the scene corresponding to more highly charged elements and darker regions corresponding to less highly charged ones.
Typically, only one-half of the elements in a CCD array are used to collect light. In one prior art design, referred to as the interline transfer CCD, rows (or columns) of photosensitive elements are alternated with rows (or columns) of non-sensitive elements. The latter serve as xe2x80x9cconduitsxe2x80x9d to transfer the electronic charges from the photosensitive elements to an output buffer, or readout register.
The acquisition of an image in a typical CCD array occurs over several phases. To begin, the photosensitive elements are grounded or xe2x80x9cresetxe2x80x9d to remove all previously accumulated charge. Once this is completed, light (or other energy) from a lens begins to accumulate, forming an image. Since CCD devices are not typically equipped with mechanical shutters, accumulation continues until the charges built-up in each of the photosensitive elements are transferred to the corresponding non-photosensitive elements. The period following the reset and before the transfer is, accordingly, referred to as the integration or shuttering period.
According to prior art image acquisition system designs, once the integration period ends, the charges transferred to the non-photosensitive elements are immediately and quickly shifted across (or down) each row (or column) of non-photosensitive elements to the readout register. From there, they are loaded into to the memory of the host device (e.g., video workstation) into which the CCD has been incorporated.
Another type of prior art CCD array, the full frame CCD, is configured differently from the interline CCD, but is operated similarly. Rather than having interleaved rows of photosensitive and non-photosensitive elements, the photosensitive elements are disposed together on one-half of the array; the latter, on the other half. Once the integration period ends, the charges accumulated in the photosensitive elements are shifted from neighbor to neighbor until they occupy corresponding elements on the non-photosensitive half, whence they are transferred via the readout array to the host memory.
Regardless of which prior art CCD design is used, prior art image acquisition system designs require that memory be dedicated in the host device to receive the image data as it streams from the readout register. Sufficient bandwidth must also be provided on the bus or other communications path to insure that the streaming data is not lost en route to the memory. Still further, because the CCD devices typically operate with their own internal clocks, the host must be designed to accommodate asynchronous transfers from the readout register.
For example, conventional machine vision systems using a CCD camera based upon one of the popular video standards (i.e. RS170 or CCIR) have no control over when the image data is output by the CCD""s, since they are constantly outputting new video information after each vertical blanking interval. In applications that use electronic shuttering, on the other hand, the host must be ready to accept the video information immediately following the termination of the integration period, since there exists no mechanism which would allow the readout of the data to be postponed.
An object of this invention is to provide improved methods and apparatus for image acquisition and, more particularly, improved methods and apparatus using charge coupled devices to acquire images.
Another object is to provide such methods and apparatus as reduce the resources required for, and costs associated with, image acquisition.
Still another object of the invention is to provide such devices and methods as permit greater control of image acquisition.
Yet still another object of the invention is to provide such methods and apparatus as are adapted to use in multicamera environments, e.g., where multiple images must be acquired synchronously or asynchronously.
The foregoing are among the objects attained by the invention, which provides methods and apparatus for image acquisition that take advantage of the inherent storage capability of charge coupled device (CCD) devices. By separating the integration and readout functions of such devices, the invention permits image acquisition apparatus (e.g., video camera) or systems into which they are incorporated (e.g., machine vision systems) to control the flow of image information, e.g., to reduce the bus and memory resources required to carry it, to time its arrival with other information or images, and/or to coordinate its arrival with the availability of processing resources.
Thus, in one aspect, the invention utilizes a charge coupled device that includes a photosensitive region, with photosensitive charge collection sites, and a non-photosensitive region, with non-photosensitive readout sites. The readout sites are coupled directly (e.g., as in an interline format) or indirectly (e.g., as in a frame transfer format) to the charge collection sites. The photosensitive region responds to an applied transfer signal by transferring charge accumulated on the collection sites to the corresponding readout sites. The non-photosensitive region responds to a read signal, applied independently of the transfer signal, by outputting (e.g., to a host camera or image acquisition system) charges in the readout sites.
Unlike prior art acquisition devices, in which initiation of a transfer cycle leads directly to the output of image information from the readout sites, an acquisition device according to the invention executes transfer and readout functions independently. Thus, although the transfer and readout signals may be applied substantially simultaneously (thus, emulating prior art systems), they need not be. By delaying application of the readout signal, image information in the readout sites can be retained until the host is ready to receive it.
The apparatus further includes a control unit that responds to requests, e.g., from a host system, by generating and applying to a head unit signals for acquiring an image. The head unit responds to those signals by acquiring image information output from the CCD and generating a video signal for application to the control unit and/or the host.
Related aspects of the invention provide an image acquisition apparatus as described above in which the acquisition signal generated by the control unit include a shutter pulse that defines at least a duration of charge integration by the CCD, and a readout signal that causes charges on the CCD readout sites to be outputted, e.g., to the head unit. The head unit can respond to such a shutter pulse by applying to the CCD a reset signal that clears its charge collection sites. The head unit can subsequently apply a transfer signal to the charge collection sites, causing them to transfer their charge to the readout sites. Application of the transfer signal can be timed to effect an integration period in accord with that defined by the shutter pulse. The head unit can respond to a readout signal from the control unit by applying to the CCD a read signal that causes the charges to be transferred from the non-photosensitive sites to a readout register, buffer or other store.
Still further aspects of the invention provide an image acquisition apparatus as described above in which multiple head units are provided, each associated with a respective CCD. The control unit of such can apparatus can effect synchronous image acquisition from those head units by applying shutter pulses substantially concurrently to them. Alternatively, the images can be acquired asynchronously via application of shutter pulses at different times. Whether image acquisition is synchronous or asynchronous, the control unit can control image transfer from the head units by applying readout signals to them independently of the application of the shutter pulses. Thus, for example, in response to requests from the host, the control unit can cause images acquired simultaneously by multiple head units to be output to the host memory in any desired order.
In still further aspects, the invention provides methods for image acquisition paralleling operation of the devices and apparatus described above.
Systems in accord with the invention have a many advantages over the prior art. By taking advantage of the inherent storage capability of CCD readout sites and separating their integration functions from the readout functions, the invention provides greater control of, and reduces the resources required for, the image acquisition process. In machine vision, for example, such systems are well suited to those applications which require that the acquisition of images overlap the inspection process. Here, inspection of an image can be completed without diverting processor resources (e.g., memory and bus cycles) to acquisition of a subsequent image. Instead, the host can instruct the control unit simply to acquire the subsequent image and to defer resource-consuming transfers until processing of the prior image is completed.
Moreover, in applications requiring acquisition of multiple images, systems according to the invention can utilize a single video path (e.g. amplifiers, D.C. restoration circuits, anti-aliasing filters, A/D converters) efficiently among multiple head units. For example, a control unit can apply integration pulse signals simultaneously to the multiple head units, yet, apply the readout signals serially so that only a single video path is required to condition the resultant image signals for transfer to the host. This contrasts with prior art image acquisition technology, which would require a unique datapath for each camera head unit.